1. Field of the Invention
The present invention relates to an optical fiber, more particularly relates to an optical fiber suitably used for wavelength division multiplexing (WDM) transmission and an optical signal transmission system (optical signal transmission line) using that optical fiber.
2. Description of the Related Art
Increasing the transmission capacities in optical signal transmission systems by using WDM transmission is being actively studied. Recently, in particular, WDM transmission systems using Raman amplification have also been studied.
Raman amplification will be explained here. In general, when light strikes glass or another substance, the molecular vibration or lattice vibration of that substance causes the generation of light of a somewhat longer wavelength than the original wavelength. This light is called Raman scattering light. When the stimulated light introduced is made more intensive, further, intensive phase-aligned Raman scattering light may be produced. This is called xe2x80x9cinduced Raman scattering lightxe2x80x9d. When the introduced signal light and the induced Raman scattering light match in wavelength, the induced Raman scattering light is changed to the same degree as the signal light and the signal light is amplified. In this way, xe2x80x9cRaman amplificationxe2x80x9d is a technique for amplification of an optical signal using the fact that light scattered by atoms making up the material, that is, a silica-based glass (induced Raman scattering light), is converted to a wavelength different from that at the time of incidence and scattered when intensive stimulated light is introduced fired into an optical fiber.
Raman amplifier uses the amplification action arising due to a non-linear optical phenomenon occurring in an optical fiber in this way.
In Raman amplification, the phenomenon is known where the maximum value of a Raman gain is obtained at a wavelength away from a stimulated light source wavelength by about 100 nm to a longer wavelength side. Using this phenomenon, it has been attempted to amplify a WDM optical signal using a plurality of stimulated light sources of different wavelengths (hereinafter referred to as a xe2x80x9cwavelength multiplexed stimulated light sourcexe2x80x9d).
Here, a wavelength band of the wavelength multiplexed stimulated light source has to be no more than about 100 nm so that the stimulated light at a longest wavelength of the wavelength multiplexed stimulated light source does not overlap the signal light at a shortest wavelength of the WDM signal.
To realize an optical transmission system using such Raman amplification, it is necessary to prevent waveform distortion from becoming irreparable in the signal light at transmission repeating points and a receiving station. Therefore, it is effective to suppress a non-linear phenomenon in the optical transmission line and reduce the cumulative chromatic dispersion in the optical transmission line.
In an optical transmission system using Raman amplification, use of a single mode optical fiber (SMF) having zero dispersion near a wavelength at 1.3 xcexcm or an optical fiber increasing in dopant for raising a refractive index of a core and thereby improved in Raman efficiency is being studied for the portion of an optical transmission line for Raman amplification.
Since SMF has a relatively large effective area (Aeff) of about 80 xcexcm2, SMF is poor in efficiency for Raman amplification. When using SMF, an optical power of at least 1 W is required in total for the wavelength multiplexed stimulated light source. This is not economical.
Further, an optical fiber increasing in dopant so as to raise a refractive index of a core has inherently large Rayleigh scattering. The Rayleigh scattering coefficient is known as an indicator expressing the magnitude of the Rayleigh scattering. The value of the Rayleigh scattering coefficient is about 0.9 xcexcm4xc2x7dB/km with the above-mentioned SMF, while is almost always more than 1.1 xcexcm4xc2x7dB/km with an optical fiber increasing in dopant to raise the refractive index of the core.
If an optical fiber having such a large Rayleigh scattering coefficient is used for Raman amplification, a phenomenon where a noise component scattered at the rear of a optical transmission line scatters to the front of the next optical transmission line and is superposed on the signal, that is, double Rayleigh scattering, is apt to occur and the signal-to-noise ratio (SNR) tends to fall.
Further, an optical fiber increasing in dopant to raise a refractive index of a core has a zero dispersion wavelength in the signal light band or stimulated light band, so was susceptible to the disadvantages of four wave mixing (FWM) etc.
Either of deterioration of the SNR or occurrence of FWM becomes a factor causing deterioration of the bit error rate (BER) at the time of WDM transmission, so it is not suitable to use an optical fiber with a large Rayleigh scattering coefficient or an optical fiber increasing in dopant for raising a refractive index of a core for the purpose of Raman amplification in a dense WDM (DWDM) transmission system where the BER has to be kept low.
That is, the above two types of optical fibers suffered from the disadvantages that they were not suitable as optical fibers for Raman amplification.
An object of the present invention is to provide an optical fiber having a small Rayleigh scattering coefficient and a high Raman efficiency.
Another object of the present invention is to provide an optical fiber not having a zero dispersion wavelength in a signal light wavelength band and stimulated light wavelength band.
Still another object of the present invention is to provide an optical signal transmission system (optical signal transmission line) performing Raman amplification by using such an optical fiber.
According to a first aspect of the present invention, there is provided an optical fiber having a refractive index profile, including at least one annular region between a center core and a cladding layer, having a Rayleigh scattering coefficient not more than 1 xcexcm4xc2x7dB/km, a first relative refractive index difference of the center core with respect to the cladding layer of a positive value of not more than 0.9%, a second relative refractive index difference of an annular region adjoining the center core with respect to the cladding layer from xe2x88x920.7% to xe2x88x920.2%, and an effective area (Aeff) not more than 60 xcexcm2.
According to a second aspect of the present invention, there is provided an optical signal transmission system having an optical signal transmission apparatus for transmitting an optical signal, a Raman amplification means for amplifying signal light transmitted from the optical signal transmission apparatus, and an optical signal reception apparatus for receiving a signal transmitted from the Raman amplification means, the Raman amplification means having an optical signal transmission line including in at least part an optical fiber having a refractive index profile including at least one annular region between a center core and a cladding layer and having a Rayleigh scattering coefficient not more than 1 xcexcm4xc2x7dB/km, a first relative refractive index difference xcex941 of the center core with respect to the cladding layer of a positive value of not more than 0.9%, a second relative refractive index difference xcex942 of an annular region adjoining the center core with respect to the cladding layer from xe2x88x920.7% to xe2x88x920.2%, and an effective area (Aeff) not more than 60 xcexcm2 and receiving signal light emitted from the optical signal transmission apparatus, a wavelength multiplexed stimulated light source for providing a plurality of stimulated light having different wavelengths, and an optical coupling means for coupling an optical signal transmitted from the optical signal transmission line and an optical signal transmitted from the wavelength multiplexed stimulated light source and performing Raman amplification by wavelength multiplexed stimulated light incident into the optical fiber in the optical signal transmission line from the wavelength multiplexed stimulated light source.